The present invention relates to a silicon nitride sintered material which has heat-shieldability, appropriate electrical conductivity, high strength and excellent corrosion resistance and which is dense, as well as to a process for production thereof.
As a member used for holding or transferring a to-be-treated material or a to-be-transferred material, used in equipment for semiconductor production, equipment for production of flat panel displays, or equipment used for hard disc production, etc., i.e. a holding or transferring member, or as a member constituting the inside of a treating chamber, there have been proposed members made of a ceramic material, for their corrosion resistance and abrasion resistance.
In general, a to-be-treated material or a to-be-transferred material, for example, a circuit-formed silicon wafer, when placed in production equipment such as mentioned above may be destroyed by discharging. Additionally, there has been a problem that the small particles present inside a treating chamber adhere electrostatically onto a holding or transferring member or a member constituting the inside of a treating chamber and consequently the particles adhere onto a to-be-treated material or a to-be-transferred material.
It is generally known that a to-be-treated material or a to-be-transferred material is preferably kept at a constant temperature during the transferring.
For the matters mentioned above, it is preferred that a ceramic material used in the above-mentioned holding or transferring member or in the above-mentioned member constituting the inside of a treating chamber is not a perfect insulator and has appropriate electrical conductivity, is low in thermal conductivity, and has heat-shieldability and heat insulation.
As the ceramic material used in such applications, there are known, for example, a material comprising an insulating ceramic (e.g. alumina, zirconia or silicon nitride) and conductive particles (e.g. SiC, TiN or ZrB2) dispersed therein.
Such a ceramic material having conductive particles dispersed is ordinarily a perfect conductive material owing to the connection among the particles and can be free from a problem of electrification; however, it has too low a resistance of less than 1xc3x97105 xcexa9xc2x7cm and has had a problem of generating a leakage current.
The ceramic material having conductive particles dispersed can be allowed to have an appropriately low electrical resistance by controlling the size and amount of the conductive particles; however, the control of the size and amount has been difficult practically.
Hence, the present applicant proposed, in Japanese Patent Application 11-176478, use of a ytterbium oxide as a sintering aid for silicon nitride to reduce the electric resistance of the silicon nitride sintered material obtained.
Further, it is described in JP-A-11-220012 that by allowing silicon nitride to contain a particular proportion of ytterbium, the silicon nitride sintered material obtained has a volume resistivity (a resistivity) of 108 to 1012 xcexa9xc2x7cm at a temperature range of 100 to 250xc2x0 C.
The ytterbium-containing silicon nitride materials mentioned above require a high temperature for liquid phase formation in sintering because they contain ytterbium as a sintering aid and must be fired at high temperatures (1,900 to 1,950xc2x0 C. in Japanese Patent Application 11-176478 and 1,900xc2x0 C. in JP-A-11-220012); however, firing at such high temperatures has had a problem of striking particle growth and high thermal conductivity (79 W/mK or more in Japanese Patent Application 11-176478 and 50 W/mK or more in JP-A-11-220012).
In view of the above-mentioned problems of the prior art, the present invention aims at providing a silicon nitride sintered material which has heat-shieldability, appropriate electrical conductivity, high strength and excellent corrosion resistance and which is dense, and a process for production thereof.
According to the present invention, there is provided a silicon nitride sintered material comprising a polycrystal material having silicon nitride crystal grains and a grain boundary phase, which sintered material contains a Yb element in an amount of 2 to 30% by weight in terms of its oxide and an Al element in an amount of 1 to 20% by weight in terms of its oxide and has a thermal conductivity of 40 W/mK or less at room temperature, a resistivity of 1xc3x97105 to 1xc3x971012 xcexa9xc2x7cm at room temperature, and a porosity of 0.5% or less.
According to the present invention, there is also provided a process for producing a silicon nitride sintered material, which comprises; subjecting a raw material obtained by adding a Yb element and an Al element to a silicon nitride powder, molding to obtain a molded material, and then firing the molded material in a non-oxidizing atmosphere, wherein the silicon nitride powder contains a xcex2-silicon nitride powder in an amount of 10 parts by weight or more per 100 parts by weight of the silicon nitride powder and the firing is conducted at 1,850xc2x0 C. or below.
The silicon nitride sintered material of the present invention is a polycrystal material which contains a ytterbium element in an amount of 2 to 30% by weight in terms of its oxide and an aluminum element in an amount of 1 to 20% by weight in terms of its oxide and which has a thermal conductivity of 40 W/mK or less at room temperature, a resistivity of 1xc3x97105 to 1xc3x971012 xcexa9xc2x7cm at room temperature, and a porosity of 0.5% or less.
By using such a constitution, there can be obtained a silicon nitride sintered material which has heat-shieldability, appropriate electric conductivity, high strength and excellent corrosion resistance and which is dense; therefore, the sintered material can be suitably used, for example, as a member for use in equipment for semiconductor production or a member for mounting an electronic part(s) thereon.
The main feature of the silicon nitride sintered material of the present invention lies in that it contains a ytterbium element in an amount of 2 to 30% by weight preferably 10 to 20% by weight in terms of its oxide and an aluminum element in an amount of 1 to 20% by weight, preferably 2 to 10% by weight in terms of its oxide.
The reason is as follows. When the content of the ytterbium element is less than 2% by weight in terms of its oxide, it is impossible to obtain a sufficiently low electrical resistance; when the ytterbium element content is more than 30% by weight, sintering is difficult and, consequently, it is impossible to obtain a low electrical resistance and the bending strength obtained is low.
When the aluminum element content is less than 1% by weight in terms of its oxide, it is impossible to obtain a sufficiently low thermal conductivity; when the aluminum element content is more than 20% by weight, sintering is difficult and, consequently, it is impossible to obtain a low electrical resistance and the bending strength obtained is low.
The ytterbium element and aluminum element used in silicon nitride powder are preferably oxides in view of the availability, but may each be other compound or a metal.
By allowing silicon nitride to contain ytterbium and aluminum, the melting point of grain boundary phase can be synergistically reduced and low-temperature firing at 1,800xc2x0 C. or below becomes possible; further, by allowing silicon nitride to contain aluminum, the thermal conductivity and resistivity of silicon nitride sintered material obtained can be changed proportionally, as shown in Table 2. In other words, by allowing silicon nitride to contain not only ytterbium but also aluminum, it is possible to control the resistivity of silicon nitride sintered material.
When the ytterbium element and aluminum element used in silicon nitride powder satisfy the above requirements, it is possible to allow the silicon nitride powder to contain, as necessary, other sintering aids, for example, Y2O3 or MgO, or to contain, as necessary, a transition metal compound (e.g. Mo2C), SiC or the like to impart light-shieldability to the sintered material obtained.
Thereby, these particles are dispersed in the sintered material and light-shieldability and high strength can be imparted to the sintered material.
Further, the silicon nitride powder used in the present invention preferably contains a xcex2-silicon nitride powder (a xcex2-powder) in an amount of 10 parts by weight or more per 100 parts by weight of the silicon nitride powder.
By thus controlling the amount of the xcex2-powder contained in the silicon nitride powder as part of the starting material, at 10% by weight or more, a dense silicon nitride sintered material can be obtained even if the firing temperature is as low as 1,850xc2x0 C. or below.
The control of the amount of the xcex2-powder contained in the silicon nitride powder as part of the starting material can be conducted by adding a xcex2-powder to an xcex1-powder (this xcex1-powder originally contains a certain amount of a xcex2-phase) or using an appropriately selected xcex1-powder containing a required amount of a xcex2-powder.
The proportion of xcex2-phase in the present silicon nitride sintered material is a sum of the amount of xcex2-silicon nitride in starting material and the amount generated in sintering by transformation from xcex1-phase to xcex2-phase, and there is no restriction as to the proportion as long as the properties of the present silicon nitride sintered material are not impaired.
Next, in producing a silicon nitride sintered material of the present invention, it is important that the firing temperature is controlled at 1,850xc2x0 C. or below, preferably at 1,550 to 1,800xc2x0 C. in order to prevent an increase in the thermal conductivity of silicon nitride sintered material.
When the firing temperature is higher than 1,800xc2x0 C., it is preferred to conduct firing under a gas pressure to suppress the decomposition of silicon nitride; when the firing temperature is 1,800xc2x0 C. or below, it is preferred to conduct firing at normal pressure in a non-oxidizing atmosphere from the cost standpoint.
In the present invention, the firing may be conducted as necessary by hot pressing, hot isostatic pressing, or the like.
The silicon nitride sintered material produced by the present process is a polycrystal material having a thermal conductivity of 40 W/mK or less at room temperature, a resistivity of 1xc3x97105 to 1xc3x971012 xcexa9xc2x7cm at room temperature, and a porosity of 0.5% or less, and there can be obtained a silicon nitride sintered material which has heat-shieldability, appropriate electrical conductivity, high strength and excellent corrosion resistance and which is dense.